docs: def _get_stddevs_pga(self, C, rup):
          """
          Returns the inter- and intra-event coefficients for PGA
          """
          tau_lnpga_b = self._get_taulny(C, rup.mag)
          phi_lnpga_b = np.sqrt(self._get_philny(C, rup.mag) ** 2. -
                                self.CONSTS["philnAF"] ** 2.)
          return tau_lnpga_b, phi_lnpga_b